1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, it relates to a magnetic recording medium suitable for an audio cassette tape for metal position using a cobalt adsorbed iron oxide.
2. Description of the Prior Arts
Recently, a magnetic tape using a metallic powder as a magnetic powder which is called as a metallic tape has been proposed and used as an audio cassette tape having high performances. These tapes have a coercive force Hc of 950 to 1200 Oe and a residual magnetic flux density Br of 2400 to 3200 G which are about 2 to 3 times of those of the conventional audio cassette tapes as remarkably high performances which have not attained in the conventional ones. However, the cost is remarkably expensive since a metallic powder as pure metal is used as a magnetic medium. The metallic powder is easily oxidized whereby it is difficult to attain a technology for preventing oxidation and there is possibility to cause firing. Therefore, it is not easy to produce a metallic tape in stable and economical in a mass production.
The metallic tape has excellent MOL (maximum output level) as dynamic range but has not always high sensitivity which is the weakpoint thereof.
Such troubles can be overcome if the characteristics of the metallic tape can be provided by using an iron oxide type magnetic powder as the magnetic medium source. The high frequency maximum output level is improved by increasing a coercive force Hc which can be attained by adsorbing or doping cobalt to iron oxide powder. However, the low frequency maximum output level is inferior if the residual magnetic flux density (Br) is not increased whereby the tone quality is unbalanced. This is the reason why the conventional iron oxide type magnetic power could not be used.
In order to attain a coercive force of a tape of more than 950 Oe and a low frequency sensitivity and a maximum output level which are the same or higher than those of the metallic tape, it is necessary to have magnetic flux (.phi..sub.r) of 0.35 maxwell or higher (magnetic flux: .phi..sub.r =Br.times.sectional area of tape). Thus, in order to attain such high performances for the conventional iron oxide type magnetic recording tape, it is necessary to give a relatively thick coated layer. In the case of the audio cassette tape, the thickness of the coated layer should be usually 7.5.mu. or less in view of the structure thereof. When Br is 1800 G or higher, it is easy to attain .phi..sub.r of 0.35 maxwell or more. However, it has been difficult to give such high Br in the conventional iron oxide type magnetic recording tape.